1. Field of the Invention
The present invention relates to a photosensitive member which has a photoconductive layer containing amorphous silicon, and more particularly to a photosensitive member which has an insulation layer formed over the photoconductive layer.
2. Description of the Prior Art
For the past several years, attention has been focused on the application to photosensitive members of amorphous silicon (hereinafter referred to as "a-Si") which is produced by the glow discharge decomposition process or sputtering process. Similarly, attention has been directed to amorphous silicon-germanium (hereinafter referred to as "a-Si:Ge") having improved sensitivity in the region of long wavelengths for use in forming images by a semiconductor laser. Such promising application is attributable to the fact that for use in photosensitive members, a-Si and a-Si:Ge are exceedingly superior to the conventional selenium and CdS materials in resistance to environmetal pollution, heat and abrasion, photosensitive characteristics, etc.
However, a-Si or a-Si:Ge has the drawback of being low in dark resistivity and unusable as it is for a photoconductive layer which also serves as a charge retaining layer. It has therefore been proposed to incorporate oxygen or nitrogen into the material to improve the dark resistivity, but this conversely results in reduced photosensitivity, hence there is a limit to the content of the additive.
To overcome the drawback that the photosensitive member prepared from a-Si has a low dark resistivity and a very high rate of dark decay, it is also proposed to form an insulation layer of carbon-containing a-Si on a photoconductive a-Si layer to give improved charge retentivity (e.g. Published Unexamined Japanese Patent Application SHO No. 57-115551 and U.S. Pat. No. 4,465,750). The former publication discloses that carbon atoms are incorporated into a-Si at a high concentration of 40 to 90 atomic % (hereinafter abbreviated as "at. %"). Nevertheless, high carbon contents result in optical fatigue or reduced sensitivity, whereas improved chargeability requires a higher carbon concentration, which needs to be at least 70 at. % in some cases. Overcoat layers of such high carbon concentration are difficult to make by the common glow discharge decomposition process. The photosensitive member obtained, which has a high carbon concentration, exhibits poor adhesion to the photoconductive layer (of a-Si or a-Si:Ge), possibly creating blank streaks in the copy images obtained. Accordingly there is a limitation to the improvement of chargeability by increasing the carbon content.